1. Field of the Invention
The present invention relates to semiconductor wafer processing equipment and, more particularly, to a method and apparatus for conserving and redirecting power applied to a substrate support.
2. Description of the Background Art
Within a process chamber of a substrate processing system, a substrate, such as a semiconductor wafer, is typically supported by a substrate support or susceptor while being processed. The susceptor is in turn supported by a pedestal assembly. The pedestal assembly houses attendant components to assist in substrate processing including but not limited to power supply connections, gas conduits and the like. To facilitate processing of the wafer, the susceptor is heated to raise the temperature of the wafer during one or more of the process steps. To facilitate heat transfer to/from the wafer, the susceptor is fabricated from a ceramic material and a heating member such as a resistive heater element is embedded therein, or pressed thereagainst. Additionally, a heat sink such as a liquid cooled block may be disposed below the susceptor. The heater element is generally a coil of resistive wire or a metallized layer fabricated from a material such as tungsten. When power is applied to this wire or layer, the element generates heat that conducts through the ceramic to the wafer.
Generally the power applied to the susceptor is on the order of 1000W. During initial heating of the susceptor and wafer (referred to as ramp-up) and subsequent cooling, the susceptor may break if it does not have sufficient thermal uniformity characteristics or too much power is applied to it. That is, the thermal stresses on the susceptor material caused by the application of the power will cause the susceptor to crack, thereby necessitating its"" replacement.
Thermal uniformity of the susceptor is not only dependent upon the susceptor material, but also upon the configuration of the susceptor and its support, known as a pedestal. The industry standard for semiconductor wafers is a 200 mm diameter, thus the susceptor is slightly larger. Recent improvements in this area of technology and the natural evolution of the field is allowing for larger substrates (on the order of a 300 mm diameter). Regardless of the susceptor size, the pedestal assembly is smaller in diameter than the susceptor. As such, a certain amount of heat radiates from the backside of the susceptor and into the chamber where the susceptor overhangs its backing structure of the pedestal. This radiative heat loss increases in a radial direction as one moves away from the center of the susceptor. Additionally, the thermal gradients in the susceptor may result in similar gradients on the substrate. Non-uniform substrate temperatures result in degraded substrate conditions (poorly formed layers or variable properties in the layers or the like).
Typically, the pedestal includes a temperature regulating member, such as a resistance wire heater, fluid channels, etc., therein to raise or lower the temperature of the susceptor and thus of the wafer. Where the pedestal is immediately under the susceptor, the susceptor temperature will be correlative with the temperature of the area of the pedestal immediately behind it. Since the pedestal assembly is smaller than the susceptor, the heat flux to the overhanging region of the susceptor comes to/from the pedestal through the susceptor, or radiatively off of the side of the pedestal rather than directly from the pedestal. Thus the greatest temperature variation leading to non-uniformity occurs at the circumference of the susceptor where the susceptor and pedestal are not positioned against each other. It has been further realized that the radiative heat losses rise significantly as susceptor diameter increases where pedestal diameter is maintained constant. For example, a substantial amount of backside surface area of a 300 mm susceptor is not covered by its pedestal assembly in comparison to a 200 mm susceptor. Therefore, there is a greater amount of heat radiating from the 300 mm susceptor backside into the process chamber resulting in a heat loss condition at the susceptor and consequently, the wafer. As such, an increased amount of power (current) must be applied to the susceptor to maintain the desired operating temperature. The increase in power can exceed the design constraints of the pedestal assembly/susceptor thereby causing irreparable damage. Two possible forms of damage are the aforementioned ceramic cracking and fusing, oxidizing or general degrading of the power terminals that connect the susceptor to a power supply. Further, the overhanging portion will have a substantial temperature gradient leading to non-uniform film (layer) properties.
Therefore, there is a need in the art for a method and apparatus for conserving heat along the overhanging portion of a susceptor to maintain greater temperature uniformity across its diameter. Such apparatus would be capable of reducing the thermal gradients in ceramic susceptors in general and to a greater extent for susceptors that have enlarged radiating surfaces thereby compensating for radiative heat losses of same.
The disadvantages heretofore associated with the prior art are overcome by the present invention of an apparatus for redirecting energy applied to a susceptor of a substrate process chamber. Specifically, the apparatus is a shield comprising one or more reflector members disposed within said process chamber. The reflector members are disposed below said susceptor whereby thermal energy radiated from a backside of said susceptor is reflected back to the susceptor.
The apparatus further comprises a bracket member proximate said reflector members which more specifically is attached to a pedestal assembly disposed below said susceptor. The bracket member has an arcuate portion that is provided with a plurality of openings. Fastening means communicate with said arcuate portion openings (preferably four of each) and said pedestal assembly.
The bracket member further has a first vertical portion transitioning from said arcuate portion and a second vertical portion transitioning from said arcuate portion. A first horizontal portion then transitions from said first vertical portion and a second horizontal portion transitions from said second vertical portion. The first and second horizontal portions each have a first flange and a second flange. The first and second flanges of the first horizontal portion have a plurality of flange openings as does the first and second flanges of the second horizontal portion. Preferably, there are three openings per flange.
A first reflector member and a second reflector member are supported by said bracket member. The first reflector member and the second reflector member each have a first end and a second end. Each of said first and second ends of the first and second reflector members have a flange. The flange on the first end of the first reflector member has a plurality of openings that coincide with the plurality of openings on the second flange of the first horizontal portion of the bracket member and the flange on the second end of the first reflector member has a plurality of openings that coincide with the plurality of openings on the second flange of the second horizontal portion of the bracket member. Similarly, the flange on the first end of the second reflector member has a plurality of openings that coincide with the plurality of openings on the first flange of the first horizontal portion of the bracket member and the flange on the second end of the second reflector member has a plurality of openings that coincide with the plurality of openings on the first flange of the second horizontal portion of the bracket member. With this arrangement, the first and second reflector members are attached to the bracket member via communication of a fastening means with the coinciding plurality of openings. Such fastening means can be bolts threaded through the coinciding openings, nut and bolt pairings, rivets, welds and dowels and the like.
The first and second reflector members have one or more lift pin access holes. Preferably, there are two lift pin access holes provided on the first reflector member and there is one lift pin access hole provided on the second reflector member. Further, the first and second reflector members are fabricated from a low emissivity material such as stainless steel (which can be polished to a highly reflective condition). Also, the first and second reflector members can be annealed.
With the apparatus as described, energy conservation can be achieved in the process chamber. Energy that would have otherwise been radiated by the susceptor and absorbed by other chamber components is instead reflected back to the susceptor. As such, less power is required to maintain the susceptor at process temperatures and the likelihood of nonuniform thermal conditions across the susceptor is greatly reduced.